first, people have to care enough before they can change the world. in my experiences, many people just don't care about anything outside of their immediate surroundings, for better or worse.

second, the barrier to entry for a web 2.0 company relative to a company that will organize the world's health, hunger,thirst, etc. is a few orders of magnitude lower.

I've got a few friends with prototypes for companies that could effect serious change on the scale Umair constantly references, but they'd need several millions of dollars before having something that's even ready for mass production.

(edit: these buddies of mine, will end up working for corporate america in non-innovative companies because they just don't have the resources to be able to take their prototypes and build them into companies. One of the guys would need about 18 months and 6 full-time research technicians/machinists.)

even with PG's "be good" it's debatable how much the majority of YC companies are going to have a positive impact on the world ... but then again, is that their aim?

maybe Fred, Umair, and a few other big players idea/operations/capital wise should start a YC to attack those very problems with young, hungry, entrepreneurs. $250-$500k per entrepreneur should be fairly reasonable to get to marketable prototype/ alpha version.